At present, over 35% of the women in the United States have reached the median age of menopause, 51 years. Hot flashes (HFs) are the most common symptom of the climacteric and occur in the vast majority of postmenopausal women. Sleep disturbance has also been reported to be highly prevalent in this population. Yet, the causal links, if any, between these 2 phenomena are I not known. In the studies proposed here, we will attempt to discern the relationships among Hfs and objective and subjective sleep disturbance. In Study 1, we will record sleep and HF parameters in postmenopausal women with HFs, those without HFs, and age-matched premenopausal women without HFs. We will perform quantitative EEG analyses, use an objective test of daytime sleepiness (MSLT) and assess subjective sleep quality with established instruments. HF frequency increases with ambient temperature. If HFs produce arousals and thereby disrupt sleep, then reducing ambient temperature should improve sleep and increasing temperature should worsen it (Study 2). Increased arousal frequency has been found in postmenopausal women with HFs. If this accounts for reports of poor sleep, then experimental sleep disruption in asymptomatic women should produce reports of poor sleep, as well. In Study 3, we will use yoked groups of symptomatic and asymptomatic women and disrupt sleep of the latter group based on recordings from the former group. We will do this using a stimulus specific to HFs (ambient heating). Despite the common use of hormone replacement therapy, its effects on sleep have not been established. In Study 4, we will systematically manipulate ambient temperature during sleep in symptomatic women before and during estrogen replacement and in a placebo-control group. In Study 5, we will determine the effects of elevated sympathetic activation on HFs and sleep using a stimulus that does not, by itself, disrupt sleep (orthostasis).